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Researchers are turning the Santa Cruz River red. Here's why

After running dry for decades, the Santa Cruz River in Tucson has been revived with treated wastewater.

The river will flow red while researchers in Pima County try to learn more about it.

The treated water has received high marks from the Environmental Protection Agency.

But some chemicals, such as pharmaceuticals and cosmetics, are difficult to remove entirely. On Tuesday researchers at the University of Arizona and the U.S. Geological Survey will pour red dye into the Santa Cruz to learn how the water moves, which will aid them in future studies.

“The main reason for putting the dye into the water, is to track that same section of water, over time," said Joseph Cufarri, of Pima County Flood Control. "So, as it moves downstream, the researchers don’t want to be testing different portions of water and get inaccurate results.”

Cufarri said the dye is non-toxic and has been used in similar studies across the country.

Ron Dungan was a senior field correspondent at KJZZ from 2020 to 2024.